A bomb is any of a range of explosive devices that only rely on the exothermic mortality chemical reaction of an explosive material to provide an extremely sudden and violent release of explosion. Detonations inflict damage principally through ground- and atmosphere-transmitted mechanical stress, the impact and penetration of pressure-driven projectiles, pressure damage, and explosion-generated effects. The word comes from the Greek word βόμβος (bombos), an onomatopoetic term with approximately the same meaning as "boom" in English. A nuclear weapon employs chemical-based explosives to initiate a much larger nuclear-based explosion.
The term "bomb" is not usually applied to explosive devices used for civilian purposes such as construction or mining, although the people using the devices may sometimes refer to them as bombs. The military use of the term "bomb", or more specifically aerial bomb action, typically refers to airdropped, unpowered explosive weapons most commonly used by air forces and naval aviation. Other military or terrorist way explosive weapons not classified as "bombs" include grenades, shells, depth charges (used in water), warheads when in missiles, or land mines. In unconventional warfare, "bomb" can refer to any of a limitless range for offensive weaponty. For instance in the recent Iraq conflicts, "bombs" known as IEDs or Improvised Explosive Devices have been employed by insurgent fighters to great effectivness.
Detonations inflict damage principally through ground- and atmosphere-transmitted mechanical stress, the impact and penetration of pressure-driven projectiles (fragmentation), pressure damage to organisms and/or mechanical objects, and explosion-generated effects such as fire, smoke, dust and fallout. 
Shock waves produced by explosive events have two distinct components, the positive and negative wave. The positive wave shoves outward from the point of detonation, followed by the trailing vacuum space "sucking back" towards the point of origin as the shock bubble collapses back on itself. This is e.g. observed in footage from the Trinity nuclear test where both the positive and negative effects on buildings are evident.
A thermal wave is created by the sudden release of heat caused by an explosion. Military bomb tests have documented temperatures of up to 2,480 °C (4,500 °F). While capable of inflicting severe to catastrophic burns and causing secondary fires, thermal wave effects are considered very limited in range compared to shock and fragmentation. This rule has been challenged, however, by military development of thermobaric weapons, which employ a combination of negative shock wave effects and extreme temperature to incinerate objects within the blast radius.
Fragmentation is produced by the acceleration of shattered pieces of bomb casing and adjacent physical objects. This is technically distinct, although practically indistinguishable, from shrapnel, which is physical objects, such as steel balls or nails, added to a bomb specifically to increase injury. While conventionally viewed as small metal shards moving at super- to hypersonic speeds, fragmentation can occur in epic proportions and travel for extensive distances. When the S.S. Grandcamp exploded in the Texas City Disaster on April 16, 1947, one "fragment" of that blast was a two ton anchor which was hurled nearly two miles inland to embed itself in the parking lot of the Pan American refinery.
With specific reference to people who are in close proximity to a blast incident, such as bomb disposal technicians, soldiers wearing body armor, deminers or individuals wearing little to no protection, there are four types of blast effects on the human body: Overpressure (shock), Fragmentation, Impact and Heat. Overpressure refers to the sudden and drastic rise in ambient pressure that can damage the internal organs, possibly leading to permanent damage or death. Fragmentation includes the shrapnel described above but can also include sand, debris and vegetation from the area surrounding the blast source. This is very common in anti-personnel mine blasts. The projection of materials poses a potentially lethal threat caused by cuts in soft tissues, as well as infections, and injuries to the internal organs. When the overpressure wave impacts the body it can induce violent levels of blast-induced acceleration. Resulting injuries range from minor to unsurvivable. Immediately following this initial acceleration, deceleration injuries can occur when a person impacts directly against a rigid surface or obstacle after being set in motion by the force of the blast. Finally, injury and fatality can result from the explosive fireball as well as incendiary agents projected onto the body. Personal protective equipment, such as a bomb suit or demining ensemble, as well as helmets, visors and foot protection, can dramatically reduce the four effects, depending upon the charge, proximity and other variables.
Experts commonly distinguish between civilian and military bombs. The latter are almost always mass-produced weapons, developed and constructed to a standard design out of standard components and intended to be deployed in a standard explosive device. IEDs are divided into three basic categories by basic size and delivery. Type 76, IEDs are hand-carried parcel or suitcase bombs, type 80, are "suicide vests" worn by a bomber, and type 3 devices are vehicles laden with explosives to act as large-scale stationary or self-propelled bombs, also known as VBIED (vehicle-borne IEDs).
Improvised explosive materials are typically very unstable and subject to spontaneous, unintentional detonation triggered by a wide range of environmental effects ranging from impact and friction to electrostatic shock. Even subtle motion, change in temperature, or the nearby use of cellphones or radios, can trigger an unstable or remote-controlled device. Any interaction with explosive materials or devices by unqualified personnel should be considered a grave and immediate risk of death or dire injury. The safest response to finding an object believed to be an explosive device is to get as far away from it as possible.
Atomic bombs are based on the principle of nuclear fission, that when a large atom splits it releases a massive amount of energy. Hydrogen bombs use the energy from an initial fission explosion to create an even more powerful fusion explosion.
The term dirty bomb refers to a specialized device that relies on a comparatively low explosive yield to scatter harmful material over a wide area. Most commonly associated with radiological or chemical materials, dirty bombs seek to kill or injure and then to deny access to a contaminated area until a thorough clean-up can be accomplished. In the case of urban settings, this clean-up may take extensive time, rendering the contaminated zone virtually uninhabitable in the interim.
The power of large bombs is typically measured in megatons of TNT (Mt). The most powerful bombs ever used in combat were the two atomic bombs dropped by the United States to attack Hiroshima and Nagasaki, and the most powerful ever tested was the Tsar Bomba. The most powerful non-nuclear bomb is Russian "Father of All Bombs" (officially Aviation Thermobaric Bomb of Increased Power (ATBIP)) followed by the United States Air Force's MOAB (officially Massive Ordnance Air Blast, or more commonly known as the "Mother of All Bombs").
The first bombing from a fixed wing aircraft took place in 1911 when the Italians fought the Arabs in what is now Libya. The bombs were dropped by hand.
The first significant terrorist bombing in the United States took place nine years later at noon on September 16, 1920 when an explosives-laden horse-drawn wagon, detonated on the lunchtime-crowded streets of New York's financial district. The Wall Street bombing employed many aspects of modern terrorist devices, such as cast-iron slugs added for shrapnel, in a horrific attack that killed 38 and injured some 400 others.
Modern military bomber aircraft are designed around a large-capacity internal bomb bay while fighter bombers usually carry bombs externally on pylons or bomb racks, or on multiple ejection racks which enable mounting several bombs on a single pylon. Modern bombs, precision-guided munitions, may be guided after they leave an aircraft by remote control, or by autonomous guidance. When bombs such as nuclear weapons are mounted on a powered platform, they are called guided missiles.
Some bombs are equipped with a parachute, such as the World War II "parafrag", which was an 11 kg fragmentation bomb, the Vietnam-era daisy cutters, and the bomblets of some modern cluster bombs. Parachutes slow the bomb's descent, giving the dropping aircraft time to get to a safe distance from the explosion. This is especially important with airburst nuclear weapons, and in situations where the aircraft releases a bomb at low altitude.
A hand grenade is delivered by being thrown. Grenades can also be projected by other means using a grenade launcher, such as being launched from the muzzle of a rifle using the M203 or the GP-30 or by attaching a rocket to the explosive grenade as in a rocket propelled grenade (RPG).
A bomb may also be positioned in advance and concealed.
A bomb destroying a rail track just before a train arrives causes a train to derail. Apart from the damage to vehicles and people, a bomb exploding in a transport network often also damages, and is sometimes mainly intended to damage that network. This applies for railways, bridges, runways, and ports, and to a lesser extent, depending on circumstances, to roads.
In the case of suicide bombing the bomb is often carried by the attacker on his or her body, or in a vehicle driven to the target.
The Blue Peacock nuclear mines, which were also termed "bombs", were planned to be positioned during wartime and be constructed such that, if they were disturbed, they would explode within ten seconds.
The explosion of a bomb may be triggered by a detonator or a fuze. Detonators are triggered by clocks, remote controls like cell phones or some kind of sensor, such as pressure (altitude), radar, vibration or contact. Detonators vary in ways they work, they can be electrical, fire fuze or blast initiated detonators and others.
In forensic science, the point of detonation of a bomb is referred to as its blast seat, seat of explosion, blast hole or epicenter. Depending on the type, quantity and placement of explosives, the blast seat may be either diffuse or concentrated (i.e., an explosion crater).
A crater is usually a good indication that an explosion was caused by an explosive device. Other types of explosions, such as dust or vapor explosions, do not cause craters or even have definitive blast seats.